1. Field of the Invention
The present invention relates to a technology for preventing an abnormal image due to abnormal fluidity of a developer.
2. Description of the Related Art
In image forming apparatuses such as copiers, facsimiles, printers, or printing machines, an image visualizing process is performed on an electrostatic latent image carried on a photosensitive element, which is a latent-image carrier, using a one-component developer or a two-component developer.
Some of developing devices are configured to carry the one-component developer using nonmagnetic toner or magnetic toner contained in a container by a developer feed member made of foamed polyurethane, and to feed the developer to a developing sleeve used for the image visualizing process.
Japanese Patent No. 3320954 discloses an invention in which a developer carried on a developing sleeve is controlled such that a layer thickness of the developer carried thereon is made uniform by a layer-thickness control member, which is an elastic metal thin plate, before the developer reaches a position opposed to a photosensitive element.
The image visualizing process is roughly divided into a two-component developing method and a one-component developing method depending on how toner is charged. The two-component developing method uses frictional charge due to stirring and mixing of toner and carrier, and the one-component developing method uses application of charge to toner without using carrier. The one-component developing method is further divided into a magnetic one-component developing method and a non-magnetic one-component developing method according to whether a magnetic force is used for retention of toner on a developing roller.
Up to now, the two-component developing method is used in many copiers or copier-based multifunction products of which high-speed performance and high image reproducibility are demanded, because of requirements such as toner charge stability, good charge rising property, and long-term stability of image quality. Meanwhile, the one-component developing method is used in many compact printers and facsimile devices of which space saving and cost reduction are demanded.
In either developing method, colorization of output images is advancing in recent years, and thus, requests for higher image quality and stability of image quality are increasing today more than ever.
To achieve such higher image quality, an average particle size of toner is decreased, and square portions of toner particles quite often tend to be smoothed. Because of this, the toner is becoming more spherical.
In the developing device, however, as explained above, the layer-thickness control member controls the layer thickness of the toner carried on the developing sleeve, but small-sized toner and more spherical toner may sometimes easily slip under an edge of the layer-thickness control member.
As disclosed in Japanese Patent No. 3320954, of the toner whose thickness on the developing sleeve is controlled, some of toner particles that are not consumed for the image visualizing process are collected into a developer tank using a collecting member, are again stirred therein so as to increase a charge amount to a predetermined amount, and are again fed to the developing sleeve.
Therefore, the toner particles carried on the developing sleeve repeatedly slip under the layer-thickness control member. However, slidable friction due to repetition of the slipping may sometimes cause particles as a fluidization promoter being an external additive of the toner to be scraped off, or shape deformation of the toner particles due to the same factor may sometimes cause their original functions to be increasingly degraded.
Reasons for the cases can be considered as follows.
Of the toner particles having reached a position of the layer-thickness control member, some toner particles having a height of a toner chain that exceeds the layer thickness controlled by the layer-thickness control member collide against the layer-thickness control member. The collision may cause the shapes of the toner particles to be deformed or to be partially chipped, which makes it impossible to obtain a charge amount such that the charge amount is supposed to be obtained based on regularly shaped toner particles.
Further, the same goes for a case in which the toner particles undergo high frictional force due to scraping force received from the collecting member when the toner particles are collected from the developing sleeve.
As explained above, the degradation of the toner particles such as the shape deformation and the partial chipping causes encapsulated additives and waxes to be exposed. At this state, a predetermined charge amount cannot be obtained because the condition of charging the surface of toner is changed.
When the toner particles are degraded, especially, the particles as the fluidization promoter are removed from toner particles, fluidity of the toner particles is degraded, which causes the degraded toner particles to accumulate on and to be condensed on the surface of the layer-thickness control member. Consequently, the condensed toner particles adhere on the surface thereof. This case leads to production of a portion in which the toner particles do not adhere on the surface thereof and of a portion in which the toner particles adhere thereon. As a result, the distribution of the layer thickness controlled by the layer-thickness control member varies depending on the portions, and thus, the layer thickness cannot be controlled to a uniform one.
A blade is used as the layer-thickness control member, and streaks and dents are produced on a surface of the blade, in particular, at a process of rolling the blade, and further scratches or irregularities are sometimes produced thereon. If these irregularities are produced, toner particles may easily enter the streaks or the like at the same position as the irregularities in a width direction of the layer-thickness control member. Therefore, this state is continued, and the toner particles are eventually condensed to adhere on the portion.
If the toner partially adheres on the layer-thickness control member in the width direction, a portion on which the toner adheres may be different from normal layer-thickness control dimensions. This causes the width direction of the layer-thickness control member, or the distribution of the layer thickness in an axial direction of the developing sleeve, to be made nonuniform.
Therefore, Japanese Patent Application Laid-open No. H07-117267 proposes a method of eliminating scratches and irregularities on which toner particles tend to be accumulated by polishing the surface of a blade used as the layer-thickness control member.
The case in which the surface of the blade is polished to eliminate the scratches and the irregularities causes processing cost to be increased, because one process is increased in the manufacturing process that results in two processes, as compared with a case in which secondary processing such as polishing is not performed.
A factor why the toner particles are easily accumulated in the irregularities produced on the surface of the blade is caused by not only the blade itself but also the toner itself that easily moves. Specifically, it is also considered that an accelerated agglomeration degree of toner that affects mobility of the toner is also caused to slip the toner particles through a nip portion between the blade and the developing sleeve.
If the accelerated agglomeration degree of toner is higher, the toner is more difficult to move, while if the accelerated agglomeration degree of toner is lower, then the toner more easily moves. Therefore, when the accelerated agglomeration degree of toner is low, the toner easily enters into a narrow portion, and also easily passes through the nip portion with the blade. This causes the slipping to be easily repeated, which causes degradation of the toner to easily occur.